Dilemmas in Lab Testing

  • C diff testing
  • FENa in AKI
  • How to work up an elevated aPTT
  • Which coag tests for oral anticoagulants?
  • IVC diameter on US to evaluate intravascular volume status
C DIFF PCR – DO ALL POSITIVES NEED TREATMENT?
  • Case:  woman treated with full course metronidazole with diarrhea.  Repeat C diff PCR is positive.  WBC normal.
  • Available tests:  culture, ELISA for glutamate dehydrogenase, ELISA for toxin A or B, cytotoxic assay for cultured C diff, PCR.
  • IDSA testing recommendations (2010)
    • only test on loose stool
    • stool culture most sensitive
    • testing toxogenic culture is most specific
    • ELISA suboptimal
    • glutamate dehydrogenase screen followed by cytotoxic assay recommended by IDSA
    • PCR is rapid, sensitive and specific (but more data needed)
  • C diff rates skyrocketed in 2012 when PCR testing became widely available – but was this CLINICAL C diff?  No.
  • Polage CR, et al.  Overdiagnosis of C diff Infection in the Molecular Test Era (2016)
  • EIA toxin negative patients behave clinically the same regardless of whether they are PCR + or -.
  • Conclusion:  Treat the patient, not the PCR.  If you’re concerned that the PCR is false pos, get the EIA toxin assay.
FRACTIONAL EXCRETION OF SODIUM:  SHOULD I GIVE MY PATIENT IVF WITH A FENA < 1?
  • Outpatient:  prerenal causes 70%.  Inpatient:  35-40%.
  • Routinely throwing IVF at people worsens outcomes, so don’t do it.
  • FENa tells us:  what are our renal tubules doing with Na?
    • But any drug that affects the renal tubules will screw up your FENa.
  • Low FENa (< 1%) :  90% intravascular depletion, but…10% include oliguric pts with ATN, contrast-induced nephropathy, pigment nephropathy, obstructive nephropathy, low effective circulating volume, non-oliguric renal parenchymal injury
  • FENa > 2%:  90% ATN.  But 10% of prerenal patients, normal renal function with high NaCl intake, volume depleted patient with CRF, volume depletion with Na-altering meds, vomiting or NGT suctioning (to spill HCO3 you’ll spill Na with it).
  • FEUrea:  35% on the way down with oliguria
    • Well-hydrated pt’s have FEUrea 45-50%
  • Have to have a normal GFR at baseline, AKI, oliguria, no Na-altering meds, and no mimickers – OK to use FENa.
 
ELEVATED aPTT AND POSITIVE ANTICARDIOLIPIN ANTIBODIES – TO ANTICOAGULATE OR NOT?
  • Causes of elevated aPTT:  heparin use, delays in running specimen, clotting factor deficiency (vWF, 8, 9, 11, 12), clotting factor inhibitor (same), APLAb, elevated Hct.
  • Should repeat the aPTT – do a 1:1 mixing study.
  • What are the APLAb?  Lupus anticoagulant studies (dilute RVV, dilute thromboplastin time, etc.), anti-B2-GLP1, anticardiolipin antibody.
  • What about incidental positive APLAs?
    • ACLAs in 6.5% blood donors, but none with thrombosis at 3 years
    • Pregnant women often have ACLAs and Lupus Anticoag (18 and 12% respectively)
    • Triple positive patients 25% risk of clot at 5 years – and ASA prophylaxis does not help.
      • Now followed for 13 years – 2.3% risk per patient year.
HIGH INR IN A PATIENT ON DABIGATRAN – DOES IT MEAN THEY’LL BLEED?
  • Case:  Pt with ortho fx, last dabigatran dose 24 h ago, elevated INR – when okay to go to OR?
  • Why would we ever want to monitor levels of the TSOAs (target-specific oral anticoagulants)?
    • Urgent/emergency surgery
    • Suspected hemorrhage
    • Concern for overdose
    • Acute thrombosis on therapy
    • Concern for compliance
    • Trauma
  • Dabigatran:  Ecarin clotting time or dilute thrombin time.  In an emergency – aPTT.
  • Rivaroxaban and other “bans”:  anti-factor Xa level (normalized for the drug).  In an emergency – PT
  • Ciraparantag – new magic bullet that binds to all the TSOACs, but it’s not available yet.
  • Other treatment options:  prothrombin complex concentrates, activated prothrombin complex concentrates, recombinant factor VIIa, plasma, dialysis, transexamic acid.
DILATED IVC – TO DIURESE OR NOT TO DIURESE?
  • Does IVC diameter correlate with intravascular volume?
  • IVC measured at the junction of the hepatic vein enters the IVC
  • IVC compressibility index:  IVCmax – IVCmin / IVC min (??)
  • False positives (enlarged IVC):  pt positioning, mechanically ventilated, large BMIs, males, dialysis patients, tricuspid disease or surgery, tech, young athletes (esp swimmers).
  • Some IVC diameters and CVPs:
    • IVC diameter < 1.5 = CVP < 5 mmHg
    • IVC diameter > 2.0 = CVP > 10 mmHg
    • IVC Comp Index <25-50% = CVP > 10
    • > 2cm and <40% = CVP > 15% **
  • Critical Care 2015; 19:400ff – IVC min was the best predictor of IVF responsiveness in patients.
  • Studies have not correlated IVC diameter with clinical outcomes (yet).
  • Conclusion:  IVC diameter can help in those difficult-to-determine volume situations.
By the way, obesity FALSELY SUPPRESSES BNP.
Morgan Freeman has a falsely high BNP – African-Americans, old age, diabetes mellitus (Freeman has T2DM).
 From Rocky Mountain Hospital Medicine conference, 2016, Pell
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